UK Customers with Missing City — AND, OR, NOT in SQL

The problem

Brightlane's UK data-enrichment team is prioritising incomplete profiles for an outreach effort to collect missing location data.

Write a query to return the ID and name of every UK customer who has no city on file.

Assumptions:

The customers table contains every customer Brightlane has on file.
UK customers are identified by country = 'GB'.
A missing city is stored as NULL, not as an empty string.

Output:

One row per qualifying UK customer, with columns id and name.

Schema · ecommerce 5 tables

categories

id integer

name text

parent_id? integer

products

id integer

name text

category_id integer

price numeric

stock_qty integer

attributes? jsonb

order_items

id integer

order_id integer

product_id integer

quantity integer

unit_price numeric

customers

id integer

name text

email text

city? text

country text

created_at timestamptz

is_active boolean

orders

id integer

customer_id integer

ordered_at timestamptz

status text

total_amount numeric

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Worked solution Try it yourself first

Solution query

SELECT
  id,
  name
FROM
  customers
WHERE
  country = 'GB'
  AND city IS NULL

The shape

A country equality and an IS NULL test joined by AND — both have to hold on the same row for it to land in the outreach list. The result is a single row: Ola Nelson, the one UK customer with no city recorded.

Clause by clause

SELECT id, name returns just the two columns the outreach effort needs to address each profile.
FROM customers reads the customer table. The filter narrows the population from there.
WHERE country = 'GB' AND city IS NULL is the compound condition. country = 'GB' keeps UK rows. city IS NULL keeps rows where the city is absent. The AND requires both to be true on the same row, so a UK customer with a city on file is dropped, and a US customer with no city is also dropped. Only the UK customers with missing city values reach SELECT.

Why `IS NULL` and not `city = NULL`

A comparison with = against NULL doesn't return true or false. It returns unknown, because PostgreSQL can't decide whether a missing value equals anything. WHERE keeps only rows where the condition evaluates to true, so an unknown row is silently dropped — and every NULL row goes through that branch. The query runs, returns zero rows, and looks like "there are no UK customers with a missing city" when really the test never matched anything.

IS NULL is the operator that exists for exactly this case. It doesn't compare values; it asks a direct question — is this value absent? — and always returns true or false. That's the only correct way to test for a missing value in PostgreSQL.

You practiced combining a value-equality test with an IS NULL test using AND. NULL filters mix freely with regular conditions inside a single WHERE — the only rule is that NULL itself must be tested with IS NULL, never with =.

Return the ID and name of every UK customer who has no city on file

The shape

Clause by clause

Why `IS NULL` and not `city = NULL`

Reading explains SQL. Writing it, over and over with instant feedback, is what makes you fluent.

The shape

Clause by clause

Why IS NULL and not city = NULL

Reading explains SQL. Writing it, over and over with instant feedback, is what makes you fluent.

Why `IS NULL` and not `city = NULL`